Key components of the host innate immune response are pattern recognition receptors (PRRs) that sense conserved structural components of viral pathogens and subsequently activate signaling cascades leading to effective antiviral defense mechanisms. RIG-I and MDA5, members of the RIG-I-like receptor (RLR) family, are key cytosolic receptors for sensing various RNA viruses, including influenza virus, paramyxoviruses, and dengue virus. Upon binding of viral RNA, RLRs initiate signal transduction that results in type-I interferon (IFN) gene expression to suppress viral replication and the spread of infection. For their efficient replication, however, viruses have evolved elegant strategies to evade the IFN-mediated host immune response. Despite the recent progress in deciphering molecular components in the RLR pathway, the regulation of RLR-induced immune signaling remains largely undetermined; however, it is exactly the regulation of host immunity which dictates the outcome of the viral infection and disease. The proposed study will build upon a recent discovery made by the Gack laboratory that the cellular phosphatases PP1?/? are essential activators of RLR signal transduction. PP1?/? are recruited to RIG-I and MDA5 upon viral infection and induce RLR dephosphorylation, promoting antiviral signaling. Virus replication studies demonstrated that PP1?/? are required for an effective type-I IFN response to paramyxoviruses, influenza virus, picornaviruses, and dengue virus. Furthermore, our most recent study revealed that several members of the paramyxovirus family target PP1?/? to evade immune surveillance by MDA5. Molecular, biochemical, and cell biological approaches combined with virus infection studies will focus on defining in precise detail how the phosphatase PP1 activates RLR- mediated antiviral signaling (Aim 1). This study will also give detailed insights into the mechanisms by which viruses counteract PP1 and thereby RLR-mediated immunity, and further determine the physiological role of this novel immune escape strategy in IFN antagonism and virus replication (Aim 2). Insights gained from this study will not only illuminate new views on immune regulatory networks, but also identify novel host-virus interactions, thereby providing the foundation for developing novel therapeutic strategies for emerging virus-associated disorders.